Evidence against the involvement of cyclic GMP in the insulin-stimulation of lipoprotein lipase activity in fat cells

Stimulatory effects of adenosine, adenosine analogs and insulin on adipose tissue lipoprotein lipase activity and their prevention by phosphodiesterase inhibitors

Adenosine and its 'Ri'- and 'P'-site analogs, N6-phenylisopropyladenosine and 2'-deoxyadenosine, stimulate, like insulin, lipoprotein lipase (LPL) activity in adipose tissue, an effect which is suppressed by cycloheximide. However, adenosine and its analogs do not potentiate the effects of submaximal insulin concentrations. As addition of cyclic AMP phosphodiesterase inhibitors abolishes the LPL stimulatory effects of insulin, adenosine and its analogs, this study suggests that these LPL effects are mediated through common mechanisms which are likely a decrease in cyclic AMP and an increase in LPL biosynthesis.

A comparison of the effects of sodium nitroprusside and insulin on the control of metabolism in rat isolated adipocytes

Sodium nitroprusside, a known activator of guanylate cyclase within cells, was used as a probe to investigate the possible role of cyclic GMP in the control of metabolism within rat isolated white adipocytes. Over the concentration range 0-0.1 mM, it increased intracellular cyclic GMP concentrations up to 6-fold within 2 min. Over the same concentration range, it increased the incorporation of 14C from D-[U-14C]glucose into triacylglycerol and of L-[14C]leucine into protein. It also inhibited adrenalin -stimulated lipolysis in the cells, but had no effect on the transport of glucose into the cells. The effects of sodium nitroprusside were compared with those elicited by insulin under identical conditions, as this hormone was shown to cause a similar, but transient, rise in intracellular cyclic GMP concentrations within these cells. Nor insulin, neither sodium nitroprusside were able to increase cyclic AMP levels in adipocytes, whereas adrenalin (0.3 microM) stimulated this production. It is suggested that cyclic GMP may have a role in the control of some part of metabolism 'glucose or amino acids' in adipocytes, and that sodium nitroprusside is a useful probe to investigate this. The limitation of its use are discussed.